With an apparatus of this type, which is known from German patent specification 22 46 672, a sample is extracted via a valve from a tank, a reactor or a pipe containing the melt. For this purpose the apparatus has a moulding chamber which is connected to the chamber for working the melt via a transfer channel, there being provided a shut-off device in the transfer channel. The moulding chamber contains a displacer by means of which a partial evacuation can take place firstly by drawing back the displacer, which partial evacuation enables the sample to run into the moulding chamber. After the sample has solidified, a wall of the moulding chamber is detached and the test sample is ejected by means of the displacer. With this known device the melt in the melt working chamber cannot be prevented from coming into contact with the atmosphere. Moreover, the apparatus is very costly. Finally, it has dead spaces to a high degree. For the reasons mentioned sampling can only be effected in each case with the known apparatus at long time intervals if the melt intended for further processing is not to be impaired in quality. With modern continuous methods for the preparation and processing of plastic materials, as are used particularly in compounding or extruding plants, a constant quality control would be desirable per se, that is in so-called real-time processing, i.e. a quality control with which the individual test samples can be associated quite precisely with the melt at a specific point of time. The manufacture of actual test samples is unalterable in many fields of application; naturally it permits only a discontinuous manufacture of test samples, that is also only an intermittent extraction of melt samples.
German patent specification 884 105 discloses an injection moulding process and an apparatus for manufacturing mouldings from thermoplastic materials, in which apparatus the discharge element of a twin screw screw-type machine is followed by a transfer channel, by which machine the melt can be pressed into a mould by means of a pressure piston which is displaceable in the channel, the transfer channel being filled directly by the pressure of the melt in the machine, the pressure piston closing the moulding nozzles when the melt is pressed out into the mould.